That about covers it - BUT you also need to undo the screws holding the blower motor in and drop the blower motor.
This is how I flushed out my matrix, kept doing this until the water in the bucket was virtually clear.
Worked fine for me, lovely hot air after - the
problem will be it depends on how much sediment you have in circulation in your coolant system? This will be unknown and seems to vary from car to car. Mine blocked twice again, each time longer between blocking (months rather than weeks), but I can now get the matrix out in less than 30 mins!.
I have since done my timing belt/water pump and flushed/replaced my coolant/thermostat (keeping my original cleaned heater matrix)
Same engine as mine, I thought it was called the CUPA but apparently it's the EA288. Still learning...
www.seatcupra.net
edit:
Also to add - you don't need to drain your coolant - just be prepared (plastic sheet/cloth/shallow tray) to collect a small amount of coolant when you release the
lower pipe . No more coolant comes out. Probably only a can of coke's worth.